Solid high energy compositions, such as propellants, explosives, pyrotechnics or the like, typically include solid particulates, such as fuel particles and oxidizer particles, dispersed and immobilized in a cured binder matrix.
Typical oxidizers include commonly used oxidizers for propellant compositions, such as ammonium nitrate (AN), ammonium perchlorate (AP), hydroxylammonium nitrate (HAN), alkali metal perchlorates, alkali metal nitrates, or mixtures thereof.
Typical reactive metal fuels include those commonly used in propellant compositions such as Boron (B) or other metals such as Al, Mg, Zn, W, Zr, Ti, or mixtures thereof.
One type of elastomer binder which has considerable promise is based upon glycidyl azide polymer (GAP). GAP is a hydroxyl-terminated polyether polymer. GAP can be cured, as is conventional, with polyfunctional isocyanates to form elastomers. GAP-based elastomers used as propellant binders provide a definite energy advantage relative to polycaprolactone (PCP)-based elastomers, and polyethylene glycol (PEG)-based elastomers, and polybutadiene polymers such as hydroxyl-terminated polybutadiene (HTPB).
Previous inventors have cured boron with a mixture of GAP polymer, plasticizer, a cure catalyst, and isocyanate. Current methods of making GAP propellants that incorporate solid reactive metal fuels, and particularly boron (B), typically involve mixing the boron with GAP polymer, plasticizer, and a cure catalyst first and then adding the isocyanate last. These existing methods possess a number of limitations. Reaction mixtures of GAP and isocyanates using known effective catalysts in the presence of some commonly used propellant ingredients have a tendency to gas such that the cured propellants produced are not useable. Specifically, severe gassing results when boron is first mixed with GAP polymer and plasticizer, and then the cure catalyst [DBTDL]-isocyanate [IPDI] mixture is later added. The DBTDL is a cure catalyst, and IPDI is a curative. Other cure catalysts can be used, such as TPB, triphenyl bismuth. It is also possible to cure the polymer without using the catalyst. However, most formulators use the catalyst to ensure that the polymer cures. DBTDL makes the mixtures cure very fast, while the others are slower to cure. However, when the cure catalyst [DBTDL]-isocyanate [IPDI] mixture is added, the gassing occurs because residual acidity on the boron particles reacts with the isocyanate. The gassing results in undesirable holes and voids within the mix, and the end product is voluminous, looks like a sponge, and is not usable. It would be greatly advantageous to avoid this gassing.
Efforts toward this end have been attempted. For example, U.S. Pat. No. 4,379,903 shows a propellant binder cure catalyst in which curing of GAP/isocyanate mixtures as binders is accomplished without gassing by employing a catalyst comprising a mixture of cure catalyst such as triphenyl bismuth (TPB) and small amounts of dibutyltin dilaurate (tin salt) within the cure system. However, boron was not used in these propellants.
It is also known that a “proton sponge” can be added to the mixture of boron, GAP polymer, plasticizer and a cure catalyst and then adding the isocyanate to this mixture. This well-known proton sponge [is 1,8-bis (dimethylamino) naphthalene] will eliminate the spongy result and severe gassing if it is added to the GAP polymer and plasticizer mixture, before this mixture is added to the cure catalyst [DBTDL]-isocyanate [IPDI]. However, the proton sponge is a rather expensive ingredient that could add unknown long-term effects to the propellant formulation.
The present inventors have unexpectedly found that it is possible to pre-react the curative with a metal to eliminate the gassing reaction when the cure catalyst [DBTDL]-isocyanate [IPDI] mixture is added. This process avoids the spongy product without using the costly proton sponge.